Electronic devices such as notebook computers, desktop computers, monitors, and the like typically receive power from an AC power source. However, in most instances, the devices require DC power to operate, so the power from the AC power source must be converted to DC power. The simplest way to accomplish this is by diode rectification circuitry. In this type of circuit, diodes are positioned in a circuit so that AC current flows in only one direction, so that the output of the rectifier maintains a non-negative voltage. This method is typically the least expensive AC-DC conversion scheme, but it also creates the most noise or “pollution” on the AC power network. Such pollution occurs when a power converter is coupled to a load that is not purely resistive (e.g., reactive loads that include capacitors and inductors), causing the current drawn from the AC power source to be out of phase with the AC voltage, which may lead to increased harmonics and other undesirable effects. Therefore, if used in large numbers, devices that use this method can greatly impact the quality of the AC power line. Additionally, reactive loads cause power converters to be less efficient. Energy stored in the reactive loads results in a time difference between the current and voltage waveforms. This stored energy returns to the power source and is not available to do work at the load, so the “real power” of the circuit is less than the “apparent power.” The ratio of real power to apparent power is generally referred to as the power factor of a circuit. As can be appreciated, a circuit with a low power factor will draw greater current to transfer a given quantity of real power than a circuit with a high power factor, which translates to increased losses in power distribution systems and increased energy costs. Hence, it is desirable to provide AC-to-DC power conversion that does not have these same shortcomings.
To achieve this, a power converter that includes power factor correction (PFC) circuitry may be used. Generally, PFC circuits seek to maintain the AC current substantially in phase with the AC voltage, so that the power converter resembles a purely resistive load to the AC power source, which reduces the pollution on the AC power line and increases the efficiency of the power converter. One type of PFC circuit is generally referred to as a passive PFC circuit. Passive PFC circuits perform power factor correction with only passive components, such as inductors and capacitors. Passive PFC circuits are typically robust and effective, but it is often difficult to reduce the distortion to acceptable levels. Furthermore, since passive PFC circuits operate at the relatively low line frequency (e.g., 50 Hz or 60 Hz), the inductors and capacitors required may be large in size and costly.
Another type of PFC circuit is generally referred to as an active PFC circuit. Active PFC circuits generally have at least one power switch, and power converters that include them may be referred to as switching power converters. Switching power converters offer both compactness and efficiency in a number of different topologies that can be placed into two main categories: isolated and non-isolated converters. In non-isolated switching power converters, such as buck (reducing voltage) or boost (increasing voltage) converters, the power output is directly coupled to the power input through a power switch. In contrast, in isolated power converters, such as flyback or forward converters, the power output is isolated from the power input through a transformer, with the power switch being located on the primary (input) side of the transformer, and the load being located on the secondary side of the transformer.
An example of a prior art flyback power converter 2 is illustrated in FIG. 1. The power converter 2 includes a power switch 8 (typically a field effect transistor), coupled to an AC power source 4 through a rectifier 6 and the primary winding of a transformer 10. A rectifying diode 12 and a bulk capacitor 14 are coupled to the secondary winding of the transformer 10. When the power switch 8 is closed (i.e., the charging stage), current flows from the AC power source 4 through the primary winding of the transformer 10, and energy is stored in the transformer's magnetic field. Then, when the power switch 8 is opened (i.e., the discharging stage), the current flow from the AC power source 4 is interrupted, causing the magnetic field to collapse, which in turn causes a reversal in the direction of the magnetic field flux change. The negative flux change induces a voltage in the opposite direction from that induced during the charging stage. The term “flyback” originates from the induced voltage reversal that occurs when the AC power source 4 current is interrupted. The reversed induced voltage tries to induce a current flow through the primary winding of the transformer 10, but the open power switch 8 prevents current from flowing. With the voltage reversed, the rectifying diode 12 is now forward biased and permits current flow through it. This current flows into the bulk capacitor 14 where it may be used to drive a load 18.
By controlling the duration of the charging stage, the voltage at an output node 16 of the power converter 2 may be regulated. The regulation of the output voltage of power converter 2 may generally be accomplished by including a voltage error amplifier (VEA) 30 for sensing the difference between an output voltage feedback signal that approximates the output voltage at the load 18, and a reference voltage, and by using this difference (i.e. error voltage) to determine how to cycle the power switch 8 so as to minimize the difference. In this regard, the VEA 30 includes a comparator 32 that has an inverting input coupled to the output node 16 through a resistor 40 and a voltage divider that includes two resistors 36, 38. Furthermore, a feedback capacitor 28 may be coupled between the inverting input and output of the comparator 32 to provide for stability. The reference voltage 34 is coupled to the non-inverting input of the comparator 32, so that the output node 26 of the comparator 32 will be driven high when the output voltage feedback signal is less than the reference voltage 34. The power converter 2 also includes a pulse width modulation (PWM) controller 22 that outputs a drive signal on a switch control node 20 to open and close the power switch 8. The PWM controller 22 uses the output node 26 of the VEA 30 to form pulses that will cycle the power switch 8 in such a way as to drive the output voltage feedback signal toward the reference voltage 34. More specifically, the PWM controller 22 receives a constant frequency oscillating signal on its inverting input 23, and the output node 26 of the VEA 30 on its non-inverting input, and adjusts the duty cycle (i.e., the ratio of time that the switch 8 is closed to the switching period) of the switch control node 20 so that the output voltage feedback signal, at the inverting input, will substantially track the reference voltage 34, which functions to maintain the output node 16 of the power converter 2 at a desired level.
As discussed above, it is becoming increasingly important that power converters operate as efficiently as possible. For example, in an effort to reduce energy consumption and greenhouse gas emissions produced by power plants, the Energy Star program was developed. Generally, the Energy Star program provides incentives to manufacturers to comply with strict power consumption guidelines. For example, effective Jul. 20, 2007, power converters will be required to operate at 80% efficiency or higher for loads that range from 20% to 100% of the power converter's rated load. Additionally, power converters will be required to operate at a power factor of greater than 0.9 at loads that are at 100% of the rated load to receive the Energy Star designation.
In order to meet these and other specifications, it may be desirable to improve the efficiency of power converters. In addition to efficiency losses associated with a low power factor, the main transformer (e.g., the transformer 10) of isolated switching power converters is also a source of power loss. First, there is power lost due to the resistance of the wire windings. Unless superconducting wires are used, there will always be power dissipated in the form of heat through the resistance of current-carrying conductors. Because transformers may require relatively long lengths of wire, this loss can be a significant factor. Increasing the gauge of the winding wire is one way to minimize this loss, but only with undesirable increases in cost, size, and weight.
In addition to resistive losses, the bulk of transformer power loss is due to magnetic effects in the core. Perhaps the most significant of these core losses is eddy-current loss, which is resistive power dissipation due to the passage of induced currents through the magnetic core. Because the core is a conductor of electricity as well as being a “conductor” of magnetic flux, there will be currents induced in the core just as there are currents induced in the secondary windings from the alternating magnetic field. These induced currents tend to circulate through the cross-section of the core perpendicularly to the primary winding turns. Their circular motion gives them their unusual name: like eddies in a stream of water that circulate rather than move in straight lines. Core materials are typically fair conductor of electricity, but not as good as the copper or aluminum from which wire windings are typically made. Consequently, these “eddy-currents” must overcome significant electrical resistance as they circulate through the core. In overcoming this resistance, they dissipate power in the form of heat. Hence, the eddy-currents create a source of inefficiency in the transformer that is difficult to eliminate.
Another transformer power loss associated with the magnetic core is that of magnetic hysteresis. All ferromagnetic materials tend to retain some degree of magnetization after exposure to an external magnetic field. This tendency to stay magnetized is called hysteresis, and it takes a certain investment in energy to overcome this opposition to change every time the magnetic field produced by the primary winding changes polarity (i.e., twice per AC cycle).
Transformer energy losses tend to worsen with increasing frequency. First, a phenomenon known as the “skin effect” is a factor. The skin effect is the tendency of an AC current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the “skin” of the conductor. This effect is amplified with increasing frequencies, which reduces the available cross-sectional area for electron flow, thereby increasing the effective resistance as the frequency goes up and creating more power lost through resistive dissipation. Magnetic core losses are also increased with higher frequencies, due to the eddy currents and hysteresis effects becoming more severe. Further, energy losses in transformers tend to worsen with increased voltage on the primary windings, due to the corresponding increases in magnetic flux density swings.
It is against this background that the power converter described herein has been developed.